I thrift shop all the time. I used to sell books on Amazon dot com as a sideline. I’ve found quite a number of valuable books through the years that I later sold.
Once I found a book with a $100 bill in it. That was a nice surprise.
At a Garage Sale I found some First Edition novels from the 1930’s. Those each brought $50 and up.
And that is why I keep looking! :)
At worst, you will have found something charming that will give you joy all your life.